The molecular basis of mammary specific and hormone regulated gene expression is being studied trough analysis of cis-acting regulatory elements of the mouse whey acidic protein (WAP) gene using in vitro systems and transgenic animals. It was shown that nuclear proteins from mammary epithelial cells form a multiple nucleoprotein complex with the WAP gene promoter and upstream region (1). The WAP and alpha-lactalbumin gene promoters share sequence motifs which are recognized by nuclear proteins, suggesting that they play some role in the regulation of these genes. Using transgenic animals we could show that the WAP gene promoter can target he expression of a foreign gene to the lactating mammary gland suggesting that it contains regulatory elements governing WAP gene expression in the intact animal (2,3). Using this mammary expression system it is also possible to produce human proteins in the milk of transgenic animals which provides an alternative way to isolate rare proteins of scientific and pharmaceutical value (2,3). The structure and function of the control region of the human cytomegalovirus (HCMV) immediate early 1 (IE1) gene has been studied using in vitro systems which partially mimic the regulation observed in vivo. The IE1 enhancer dependent transcriptional stimulation in vitro is about 25-fold and involves the binding of transcription factors. The sequence structure of the enhancer region, which extends from nucleotide -65 to -530, exhibits a highly modulate complexity and is recognized by several nuclear proteins (4). Using in vitro transcription competition assays with individual target sequences for enhancer binding factors, we could show that transcription factors bird to at least five specific enhancer sequences and mediate the activity of the IE1 gene enhancer in vitro (5).